Direct-methanol fuel cells are a promising energy source for portable devices, and some breakthroughs have been made with regard to the fuel cell membrane and catalysts.
However, the efficiency of catalysts for methanol oxidation is still limited by CO poisoning, because CO is one of the carbonyl species formed during oxidation. Use of a PtRu alloy as the anodic catalyst for electrooxidation in direct methanol fuel cells has become state-of-the-art due to effective CO tolerance of this alloy. Nevertheless, a major obstacle to the successful application of the PtRu bimetallic catalyst is the dissolution of its metallic components. Ru dissolution in particular may cause detrimental effects such as severe decay of anodic activity and increase of the ohmic resistance. Also, dissolved Ru may penetrate the membrane and migrate to the cathode of a direct methanol fuel cell, where the oxygen reduction reaction (ORR) kinetics is badly inhibited in the presence of Ru even at micromolar concentrations. The dissolution of both Ru and Pt is caused mainly by the formation of oxidized species derived from water activation.
In an effort to enhance the stability of the PtRu catalyst, Zhang et al deposited Au clusters on a Pt catalyst through galvanic displacement of a Cu monolayer on Pt by Au. J. Zhang, K. Sasaki, E. Sutter, R. R. Adzic, Stabilization of Platinum Oxygen-Reduction Electrocatalysts Using Gold Clusters, Science 2007, 315, 220-222. The gold clusters helped to shift Pt oxidation potential to the positive, improving the stability of the catalyst. Other approaches to improving the stability of catalysts for methanol oxidation include PtRuNi ternary alloy and PtRuRhNi quaternary alloy nanoparticles, which exhibited enhanced electrocatalytic activity and better stability compared to a PtRu catalyst. J. Liu, J. Cao, Q. Huang, X. Li, Z. Zou, H. Yang, Methanol oxidation on carbon-supported Pt—Ru—Ni ternary nanoparticle electrocatalysts, J Power Sources 2008, 175, 159-165; K-W Park, J-H Choi, S-A Lee, C. Pak, H. Chang, Y-E Sung, PtRuRhNi nanoparticle electrocatalyst for methanol electrooxidation in direct methanol fuel cell, J. Catalysis 2004, 224, 236-242. However, there remains a need to develop new anodic catalysts with reduced dissolution of Ru and Pt, and synthetic methods for such catalysts, so as to improve the stability and performance of DMFC.